Swift Casting not working as expected
I have created my own protocol which I plan to use instead Any
But I don't work when I try to kick it out JSONSerialization.jsonObject
Here is my own protocol
public protocol StringOrNumber {}
extension String:StringOrNumber {}
extension NSNumber:StringOrNumber {}
extension Bool:StringOrNumber {}
extension Float:StringOrNumber {}
extension CGFloat:StringOrNumber {}
extension Int32:StringOrNumber {}
extension Int64:StringOrNumber {}
extension Int:StringOrNumber {}
extension Double:StringOrNumber {}
extension Dictionary:StringOrNumber {}
extension Array:StringOrNumber {}
extension NSDictionary:StringOrNumber {}
extension NSArray:StringOrNumber {}
extension NSString:StringOrNumber {}
extension NSNull:StringOrNumber {}
Here is the code I expect to work, but it doesn't
let json = try JSONSerialization.jsonObject(with: data, options: [])
if let object = json as? [String: StringOrNumber] {
// json is a dictionary
print(object)
}
However, if I try to convert this in 2 steps, it works like below
let json = try JSONSerialization.jsonObject(with: data, options: [])
if let object = json as? [String: Any] {
// json is a dictionary
print(object)
if let newObject:[String:StringOrNumber] = object as? [String:StringOrNumber] {
// json is a newer dictionary
print(newObject)
}
}
Here is an example JSON that I am reading from a file. ( It doesn't matter that you can try it yourself too )
{
"firstName": "John",
}
I don't understand why the first part of the code doesn't work and the second does ...
thank
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it is not Swift (language) specific to Apple ...
even worse if you change the data to
let d = ["first":"john", "last":"doe", "test int": 0, "test null": NSNull()] as [String:Any]
Linux version works as expected,
["test null": <NSNull: 0x0000000000825560>, "last": "doe", "first": "john", "test int": 0]
["test null": <NSNull: 0x0000000000825560>, "last": "doe", "first": "john", "test int": 0]
["test int": 0, "last": "doe", "first": "john", "test null": <NSNull: 0x0000000000825560>]
but the print on apples
[:]
[:]
{
first = john;
last = doe;
"test int" = 0;
"test null" = "<null>";
}
It looks very strange. the following code snippet explains why
import Foundation
public protocol P {}
extension String:P {}
extension Int:P {}
extension NSNull:P {}
let d = ["first":"john", "last":"doe", "test null": NSNull(), "test int": 10] as [String:Any]
print("A)",d, type(of: d))
let d1 = d as? [String:P] ?? [:]
print("B)",d1, type(of: d1))
print()
if let data = try? JSONSerialization.data(withJSONObject: d, options: []) {
if let jobject = try? JSONSerialization.jsonObject(with: data, options: []) {
let o = jobject as? [String:Any] ?? [:]
print("1)",o, type(of: o))
var o2 = o as? [String:P] ?? [:]
print("2)",o2, type(of: o2), "is it empty?: \(o2.isEmpty)")
print()
if o2.isEmpty {
o.forEach({ (t) in
let v = t.value as? P
print("-",t.value, type(of: t.value),"as? P", v as Any)
o2[t.key] = t.value as? P ?? 0
})
}
print()
print("3)",o2)
}
}
on apple it prints
A) ["test null": <null>, "test int": 10, "first": "john", "last": "doe"] Dictionary<String, Any>
B) ["test null": <null>, "test int": 10, "first": "john", "last": "doe"] Dictionary<String, P>
1) ["test null": <null>, "test int": 10, "first": john, "last": doe] Dictionary<String, Any>
2) [:] Dictionary<String, P> is it empty?: true
- <null> NSNull as? P Optional(<null>)
- 10 __NSCFNumber as? P nil
- john NSTaggedPointerString as? P nil
- doe NSTaggedPointerString as? P nil
3) ["test null": <null>, "test int": 0, "first": 0, "last": 0]
and on linux it prints
A) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019d8c40>] Dictionary<String, Any>
B) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019d8c40>] Dictionary<String, P>
1) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019ec550>] Dictionary<String, Any>
2) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019ec550>] Dictionary<String, P> is it empty?: false
3) ["test int": 10, "last": "doe", "first": "john", "test null": <NSNull: 0x00000000019ec550>]
finally , I used slightly modified JSONSerialization source code from open source distribution (to avoid conflict with Apple Foundation, I will rename the class to _JSONSerialization :-) and change the code so that it works in my playground without any warnings and errors and ...
displays the expected results :)
Why does it work now? Key
/* A class for converting JSON to Foundation/Swift objects and converting Foundation/Swift objects to JSON. An object that may be converted to JSON must have the following properties:
- Top level object is a `Swift.Array` or `Swift.Dictionary`
- All objects are `Swift.String`, `Foundation.NSNumber`, `Swift.Array`, `Swift.Dictionary`, or `Foundation.NSNull`
- All dictionary keys are `Swift.String`s
- `NSNumber`s are not NaN or infinity */
Now conditionally lowering all possible values to P works as expected
honestly try this snippet on linux :-) and on apple.
let d3 = [1.0, 1.0E+20]
if let data = try? JSONSerialization.data(withJSONObject: d3, options: []) {
if let jobject = try? JSONSerialization.jsonObject(with: data, options: []) as? [Double] ?? [] {
print(jobject)
}
}
apple prints
[1.0, 1e+20]
and linux
[]
and with a really great value will fail. this error comes from (in open source JSONSerialization)
if doubleResult == doubleResult.rounded() {
return (Int(doubleResult), doubleDistance)
}
replace it with
if doubleResult == doubleResult.rounded() {
if doubleResult < Double(Int.max) && doubleResult > Double(Int.min) {
return (Int(doubleResult), doubleDistance)
}
}
and "deserialization" works as expected (serialization has other errors ...)
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If you just want to check protocol compliance:
Use is
insteadas
if json is [String : StringOrNumber] {
print("valid")
}
else {
print("invalid")
}
If you want to use the converted type:
if let object = (json as? [String: Any]) as? [String : StringOrNumber] {
print("valid object = \(object)")
}
else {
print("invalid")
}
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